Almost every conversation about roof drainage is about gutters. The gutter is the part you can see from the driveway. But a gutter does not remove a single litre of water on its own. It is a holding tray, and everything it collects has to leave through a pipe that is roughly the width of your forearm.
The short version
- The building code allows one downpipe to serve no more than 12 metres of gutter.
- Gutters are sized for a one in twenty year downpour, on the stated condition that they are not blocked.
- Downpipes block at the top, at the bend, and underground, and each one shows different symptoms.
- CSIRO names downpipes that are not properly connected as a cause of erosion, saturation and water migrating under a building.
- A five minute hose test will tell you which section is blocked before anyone gets on a roof.
What a downpipe is actually asked to do
The National Construction Code, Housing Provisions Part 7.4 sets the rules. Clause 7.4.5 is short and specific: downpipes must not serve more than 12 metres of gutter length each. That is the code putting a hard ceiling on how much roof one pipe is allowed to drain.
Clause 7.4.3 sets the design event. Eaves gutters are sized for rainfall with a 5 per cent annual exceedance probability, and overflow measures for a 1 per cent event. In everyday terms, the system on your house was sized for a downpour with a one in twenty chance of turning up this year, with an overflow path for the one in a hundred.
And the code's explanatory note adds the condition that matters here. The system does that "provided they are not blocked".
So the design assumption is not a mostly clear downpipe. It is a clear one. A downpipe with a wad of compacted leaf at the bend is not running at reduced capacity in any predictable way. It is a partially closed valve on the only exit the roof has.
Why the gutter gets blamed
Here is the failure that sends most people looking for a gutter cleaner.
It rains hard. Water pours over the front edge of the gutter in a sheet, usually in the same spot every time. Someone gets a ladder out, looks in the gutter, and finds it reasonably clear. The conclusion is that the gutter is too small, or that it was hung wrong.
Usually neither is true. Water is arriving at the gutter faster than the downpipe is letting it out, so the gutter fills to the brim and spills. The gutter is doing its job. The exit is the problem, and the exit is not visible from a ladder unless you look straight down it.
The three places a downpipe blocks
At the top, in the outlet
The commonest one. Debris in the gutter migrates towards the outlet, because that is where the water is going. It arrives as loose leaf and gets pressed into a plug by the flow. Signs: the gutter holds water long after rain, and there is a tide mark of silt around the outlet.
At the bend, where the pipe turns
Most downpipes have an offset near the top to bring the pipe back to the wall. That bend is where anything that got past the outlet stops. It is the hardest blockage to spot, because from the roof the outlet looks clear and from the ground the pipe looks fine. Signs: water backs up in a storm but drains eventually, and the pipe sounds solid rather than hollow if you tap it.
Underground, past the base
Below ground the water joins the property's stormwater line, and that line can be blocked by roots, silt or collapse. Signs: water coming back up out of the base of the downpipe or bubbling out at ground level during heavy rain. This one is not a roof job and it is not fixed from up a ladder.
What a blocked downpipe does to a building
Two directions, and the second is the expensive one.
Backwards, into the roof
Water that cannot get down the pipe fills the gutter, and a full gutter does not just spill forwards. The NSW SES describes the result plainly in its storm advice: "Blockages can trap water and cause it to overflow under the eaves into your roof cavity." Their full home preparation advice puts clearing gutters, downpipes and drains at the top of the list.
Downwards, into the ground beside the house
This is the part that turns a maintenance job into a structural one. CSIRO's homeowner guide to foundation maintenance and footing performance is explicit about roof plumbing as a cause of water being concentrated in one small area of soil, and it names downpipes twice.
On corrosion: "Corroded guttering or downpipes can spill water to ground."
On connection: downpipes not positively connected to a proper stormwater collection system "will direct a concentration of water to soil that is directly adjacent to footings, sometimes causing large scale problems such as erosion, saturation and migration of water under the building".
A blocked downpipe produces the same outcome as a disconnected one. The water still ends up next to the footing, just by a different route. On the reactive clays through much of the Tweed valley, wetting one corner of a slab while the rest stays dry is exactly the uneven movement that cracks walls.
The five minute test you can do from the ground
Before anyone climbs anything, this narrows the problem down.
Put a hose into the gutter near the outlet and turn it on hard. Then watch the bottom of the downpipe.
Water arrives quickly and freely: the downpipe is clear and your overflow problem is somewhere else, most likely gutter fall or gutter capacity. Water arrives as a trickle or after a long delay: there is a restriction in the pipe, most likely the bend. Nothing arrives at all and the gutter fills: the outlet or the pipe is fully blocked. Water arrives but then pools or comes back up around the base: the problem is underground, past the point a roof job reaches.
That single test will tell you more than an hour of guessing, and it costs nothing.
Keeping them clear
Downpipes are not a separate maintenance item. They are the part of a gutter clean that determines whether the clean did anything, which is why gutter cleaning here includes clearing and flushing the downpipes rather than charging it as another visit. It starts from $200.
The timing that suits this coast is late winter. The Bureau of Meteorology records at Murwillumbah put August and September as the driest months at 53.7 mm and 39.0 mm, while the NSW SES runs storm season from September to April. Clear the system in the gap, and it goes into the wet months in the condition the code assumed.
When it is not a cleaning job
Some downpipes do not need clearing, they need replacing. Rust through at the base, splits at the joins, sections that have pulled away from the wall, and outlets that have corroded out of the gutter are all past the point where clearing them helps.
That is a gutter repair where the rest of the run is sound, or a gutter replacement where it is not. Rusted and damaged downpipes get replaced along with the gutters, because putting a new gutter onto a corroded downpipe just moves the problem down a metre.
Where to start
If water is coming over your gutters in a storm, do the hose test first. If it points at the pipe, First Choice Gutter Services covers Murwillumbah, Banora Point, Tweed Heads and the coast from the Northern Rivers to the Gold Coast, and quotes are free before anything starts.
Before and after photos of the work come back to you with an explanation of what caused the problem in the first place. If something is missed, First Choice will come back. Get in touch when you are ready.